Personally I wouldn't just let the tree do its thing and set seed.

Agreed, I don't.

I have a young Black Elder, are the seeds easy to propagate?
x Jackie

Seeds from black elder (assuming you mean one of the cultivars or hybrids, such as Black Lace) are either not set, are sterile or don't come true. If you do get seed and they grow, you just might get a dark leaved form, but its not particularly likely.

Speaking of which - it's cordial time!

Or tea, which is quicker! One flower head to a scant pint of boiling water, allow to brew and enjoy. I use the flowers from the wild bush though, so as not to use up the pretty pink ones on Black Lace.